1. Field of the Invention
The present invention relates to an apparatus for effecting coordinated position/force control for a manipulator and, more particularly, to an apparatus for effecting coordinated position/force control for a manipulator which is suitable for effecting force control.
2. Description of the Related Art
Hitherto, most conventional robots have been able to effect only the position control. For this reason, in order to allow a robot to move or effect a fixing operation or the like in compliance with an object, the operation is usually performed by devising a measure such as the provision of a tool clamped by the robot with a certain compliance mechanism. With this method, it is necessary to prepare various kinds of tools and replace the tools in correspondence with the operation. Consequently, the manufacturing cost is high, and the operation time for the tool replacement is needed. In addition, there are cases where position-control type robots cannot effect operations depending on the type of operations.
If robots are provided with a force controlling mechanism in the same way as a human arm, it is possible to remarkably expand the applicable range of operation for the robots. Hence, various force controlling systems have hitherto been proposed. One such example is shown in FIG. 2 (Japanese Patent Laid-Open No. 7905/1986). In this control system, the movement of a robot hand to be realized is set by means of parameters including a virtual spring constant, a coefficient of viscosity, and mass, a velocity command value in a hand coordinate system is determined, this command value is converted to a velocity command value for each joint by using an inverse-Jacobian matrix, and each joint is moved such that its velocity ultimately becomes this velocity command value. In addition, a control method shown in FIG. 3 has also been proposed (Proceedings of 19th IEEE Conf. on Decision and Control, 1980, pp. 95-100). In this case, the movement of a hand to be realized is set by a spring constant in a hand coordinate system, a torque command value for each joint necessary for this movement is determined, and each joint is driven on the basis of the command value.
In neither system of FIGS. 2 and 3, a position control loop and a force control loop are completely separated from each other, so that there has been the drawback that when the position control is effected, the force control loop affects the operation, thereby aggravating the position control accuracy. On the other hand, when the force control is conducted, the position control loop affects the operation, thereby aggravating the force control accuracy.
Furthermore, the securing of stability is also a major task in force control robots. One factor causing instability is a disturbing force. A force which is produced as a processing tool or a gripping tool connected to a terminal of a force sensor and an inertial load of a workpiece undergo movement is also input to the force sensor and constitutes a disturbing force, thereby making the control system for the force control unstable. As a result, there has been the problem that even a component which is effecting the positional control also becomes unstable.
Accordingly, a force controlling system shown in FIG. 4 has been proposed as a technique for completely separating the position control loop and the force control loop from each other (Journal of Dynamic Systems, Measurement & Control, 102, June 1981, pp. 126-133). Namely, each control mode for the position and force with respect to each component in a hand coordinate system is designated by a selected matrix, and either the position control or the force control is thereby designated completely. With this control system, however, since a changeover between the position control and the force control is discontinuous, there has been the problem that a shock is produced at the time of the changeover. The force control can be effected for the first time when a tool end is brought into contact with an object of operation. For this reason, it is preferred that the tool is moved under position control up to a starting point of the operation, and the operational mode is gradually changed over to the force control at the beginning of the contact. On the other hand, at the time of completion of the force operation, it is preferred that the operational mode is gradually changed over to the position control as the portion of contact is separated from the object of operation. With the control system such as the one shown in FIG. 4, however, it has been impossible to effect such movement.
With conventional force controlling robots, the difficulty of securing stability has also been a major drawback. Causes of instability include a delay in a spring-mass system present in a transmission system of the manipulator, as well as vibrations that are produced in that system. No consideration has been given to this respect with the conventional force controlling robots.
In addition, although robots which effect the force control in most cases perform operations with their hand portion abutting a certain external environment, there has been the drawback that if a spring constant, i.e., hardness, of the abutting object changes, the loop gain of the force loop also changes, resulting in instability. No consideration has been given to this respect as well with the conventional force controlling robots.